The present invention relates to precision pumps capable of providing a high downstream pressure.
Such pumps are used in particular in high-performance liquid-phase chromatography (HPLC), where they are used to inject one or more selected solvents into the lining, the substrate forming the stationary phase, of a chromatography installation. Since any fluctuation in pressure and/or in delivery is troublesome in this context, it is desirable to have a pump exhibiting virtually no such fluctuation. This problem is complicated by the fact that the pump must be capable of injecting a large variety of solvents and of adapting itself to linings of which the pressure drops also vary to a large extent.
Double-valve and single-piston pumps, actuated by a continuous-current motor, are known. In such cases it is a cam profile which defines a priori the top dead point and the bottom dead point of the piston as well as the relationship between the delivery time and the return time. As a general rule, such pumps produce considerable fluctuations in pressure, if not in delivery, in the chromatographic lining.
The use of pumps of the same kind but incorporating a number of pistons enables fluctuations in delivery and pressure to be reduced, but at the cost of considerable complexity and at much greater expense.